1. Field of the Invention
The present invention relates to a catalyst unit for use in reducing and removing nitrogen oxides (NOx) contained in the exhaust gas.
2. Description of the Prior Art
Heretofore, it has been known that, in order to remove Nox contained in the exhaust gas from combustion apparatuses, it is effective that such a reducing agent as ammonia is fed in gaseous phase to a highly heated portion at the outlet of the combustion apparatus or a duct for exhaust gas to reduce and remove NOx. The typical reactions which take place in the cases described above are as follows: EQU 3NO.sub.2 +4NH.sub.3 --7/2N.sub.2 +6H.sub.2 O EQU 3NO+2NH.sub.3 --5/2N.sub.2 +3H.sub.2 O
In order to satisfactorily carry out the reduction of NOx described above, the presence of catalyst is essential, and in general, a catalytic reactor is provided downstream of an ammonia pouring point in an exhaust gas duct. As catalytic substances used in reducing NOx contained in the exhaust gas by means of ammonia, there have been known, for example, titanium (Ti) and vanadium (V) according to Japanese Patent Kokai (Laid-Open) Publication No. 128680/75; titanium (Ti), wolfram (W) and one or more out of vanadium (V), molybdenum (Mo) and iron (Fe) according to Japanese Patent Kokai (Laid-Open) Publication No. 128681/75; and titanium (Ti), Vanadium (V) and molybdenum (Mo), or, iron (Fe) according to Japanese Patent Kokai (Laid-Open) Publication No. 52363/76. The above-described catalytic substances are formed into slurry or paste in the state of hydroxide or water-soluble salt, adding thereto a suitable carrier where necessary, and resultant slurry or paste is coated on a substrate of the catalyst, and thereafter dried and calcined to be attached onto the substrate for use. Finally, the catalytic substances are attached onto said substrates in the state of oxide or sulfate.
Heretofore, the states of use of catalytic units for decreasing nitrogen oxides contained in the exhaust gas include (1) an arrangement of layers of granular catalyst contained in a vessel, (2) an arrangement of a multiplicity of cylindrical catalyst units whose axes are in parallel with the flow of gas; and (3) an arrangement of a multiplicity of plate-shaped catalyst units made of metal plate or wire mesh on which a catalytic substance is attached and being parallel with the flow of gas. However, with the above-described arrangement (1), there is encountered with such a problem that, when the exhaust gas containing a large amount of soot and smoke, dust are built up in the layers of catalyst to increase loss in pressure, whereby the flows of gas within the layers of catalyst are deviated, resulting in difficult removal of soot and dust therefrom. Further, with the above-described arrangement (2), there are presented such problems that, in addition to the problem in the arrangement (1), the working surface area as the catalyst is small as compared with the volume occupied by itself. Furthermore, with the above-described arrangement (3), the dust build-up within the catalyst units is small and loss in pressure is low, but there is such a disadvantage that the catalytic substance attached to the metal plate or wire mesh tends to fall off. For instance, in the case a metal plate is used, the catalytic substance attached thereto falls off through slight deformation or contact because the outer surfaces thereof are flat and smooth. Additionally, in the case a wire mesh is used, because of low strength and rigidity thereof, the wire mesh tends to be deformed, whereby cracks are caused to the catalyst portion, resulting in fall-off of the catalytic substance.